1. Field of the Invention
The present invention relates to a digital signal processing device and an audio apparatus such as a loudness controller and a spectrum indicating apparatus using the digital signal processing device.
2. Description of the Related Art
Recently known is a DSP (digital signal processor) capable of obtaining an output equivalent to circuit operation by arithmetically processing (e.g., four arithmetic operations) a digital signal converted from an analog signal at high speeds. The DSP includes an arithmetic processing means for arithmetically processing an input digital signal. The arithmetic processing means basically includes a data memory for storing an input digital signal data and a coefficient memory for storing plural coefficient data. The signal data and the coefficient data are sequentially read from the data memory and the coefficient memory, respectively, in accordance with a program, and these data are multiplied with each other in a multiplier. A multiplication result from the multiplier is accumulated every time the multiplication is executed. The coefficient data to be stored in the coefficient memory is controlled in writing by an external microcomputer, and when a content of arithmetic processing, e.g., a filter characteristic of a signal to be obtained by arithmetic processing is intended to be changed, the coefficient data are rewritten. A processing result data from the arithmetic processing means is output through an output register.
Constitutional elements in the DSP are operated in synchronism with a clock pulse to be generated from an internal clock generator. In the case of processing data such as audio data requiring operation of many steps in a short time, a frequency of the clock pulse must be high. However, if the frequency of the clock pulse is high, the data is output from the DSP at high speeds. Accordingly, the output data cannot be directly read by a normal microcomputer, and the control of the DSP such as rewriting of the coefficient data in the coefficient memory according to a processing result from the DSP cannot be carried out.
Further, it is known that a human auditory sensitivity is represented by a Fletcher-Munson curve wherein a frequency characteristic depends on a sound pressure. That is, treble and bass are harder to listen as a sound pressure becomes smaller. To compensate this, a sound system is provided with a loudness control circuit to be operated in association with a sound level adjusting volume so that when a sound volume is small, treble and bass are intensified, while as the sound volume increases, they are flattened.
However, in the conventional loudness control circuit, the frequency characteristic compensation is simply changed in accordance with an operational position of the sound level adjusting volume. As a result, there is a problem such that when an input audio signal level is large, treble and bass are excessively intensified, while when the input audio signal level is small, treble and bass are insufficient.
Further, a spectrum indicating apparatus for indicating a frequency distribution of an input signal such as an audio signal is known from Japanese Patent Publication No. 58-43700, for example. Such a conventional spectrum indicating apparatus is provided with a plurality of level detecting means each including a BPF (band pass filter), a detecting circuit and an LPF (low pass filter) so as to correspond to a plurality of predetermined frequency bands. A signal level of each band is detected by each level detecting means, and a detection level corresponding to each frequency band is indicated in an indicator.
Accordingly, the conventional spectrum indicating apparatus requires to have a plurality of level detecting means corresponding to the number of frequency bands so as to finely indicate the frequency distribution, thus causing complication of the construction.